pgsql: Remove stale XXX comment in logical launcher

Remove stale XXX comment in logical launcher

An XXX comment in launcher.c added by commit 2af1dc89282b questioned why
the slot drop does not request disabling logical decoding. But the
conflict detection slot is a physical slot, so there is no logical
decoding state to disable. Remove that comment and clarify the
situation elsewhere.

Author: "Zhijie Hou (Fujitsu)" <houzj(dot)fnst(at)fujitsu(dot)com>
Reviewed-by: shveta malik <shveta(dot)malik(at)gmail(dot)com>
Reviewed-by: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Backpatch-through: 19
Discussion: https://postgr.es/m/TY4PR01MB177186C8AB596F30DF704ECDC94B62@TY4PR01MB17718.jpnprd01.prod.outlook.com

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/46c6fb9a82f2eaa0b56f1231e12e65af39e5ec84

Modified Files
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src/backend/replication/logical/launcher.c | 14 +++++++++-----
1 file changed, 9 insertions(+), 5 deletions(-)